Bounce e-mails - always occurs when an accepted e-mail is rejected due to any error. This serves to inform the sender of the original e-mail about the problem.

Misdirected bounce e-mails - occur, when the sender address was faked and the returned bounce e-mail therefore reaches a sender who did not send the original e-mail.
This means that the wrong person is informed of an error that was not caused by them.

MX / MX record - is short for mail exchanger, after the entry in the DNS (Domain Name Server). This entry specifies the mail server responsible for the domain.
For example, the command nslookup -q=mx rz.rwth-aachen.de can be used to query the mail domain of the IT Center of RWTH Aachen University.